Magnetic field straightener



March 12, 1968 I J. G. ARMSTRONG 3,373,389

MAGNETI C FIELD STRAIGHTENER Filed Oct. 20, 1965 4 Sheets-Sheet 1 lnvenlor JOHN G. ARMSTRONG tlorn y March 12, 1968 J. G. ARMSTRONG 3,373,339

I MAGNETIC FIELD STRAIGHTENEH Filed Oct. 20, 1965 4 Sheets-Sheet 2 Inventor OHN G. ARMSTRONG A tlorney March 12, 1968 J. G. ARMSTRONG MAGNETIC FIELD STRAIGHTENER 4 Sheets-Sheet 5 Filed Oct. 20, 1965 MNQENNMYQ Inventor OHN G. ARM$TRON6' Atlomey March 12. 1968 Filed Oct. 20, 1965 Ji 6. ARMSTRONG 3,373,389

MGNETIC F1 ELD STRAIGHTENER 4 Sheets-Sheet 4 l/(UERSZIQ) I 1 I j A l V 070-; 05 04 53 02 m asume FROM E3 REVERSAL (ma/5) c @1500 t lnmmiar JOHN Ge NQMSVWONG army United States Patent ()fitlce 3,373,389 Patented Mar. 12, 1968 3,373,389 MAGNETIC FIELD STRAIGHTENER John Graham Armstrong, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 20, 1965, Ser. No. 498,451 Claims priority, application Great Britain, Feb. 19, 1965, 7,253/ 65 8 Claims. (Cl. 335-211) ABSTRACT OF THE DISCLOSURE A magnetic field straightener of ferromagnetic material is positioned between the envelope of an electron beam tube and a cylindrical permanent magnet focusing structure. Two longitudinal sections are arranged on each side of a field reversal gap. The field straightener is supported and spaced from the envelope by non-magnetic metallic supports and is in the form of a small diameter wire helix closely wound on a non-magnetic metallic cylinder.

The invention relates to a magnet arrangement including a pair of hollow cylindrical permanent magnets aligned end to end with like poles adjacent one another so as to provide an axial magnetic field of substantially constant flux density from end to end with an abrupt reversal of direction of field at the gap or junction between the magnets. Such a magnet arrangement is applicable to the focusing of travelling wave tubes.

In reverse field magnetic focusing systems it is usually necessary to compensate at each reversal for the fact that the change in direction of the magnetic field is not abrupt. One method of compensation is to produce a peak of field strength close to the reversal on one or, preferably, on both sides of it. It is also common to use field straighteners which provide a low reluctance path for transverse magnetic fields, but are usually designed to give as small as possible a reduction in axial field strength. Field straighteners may be provided either by means of transverse soft iron plates or by means of a thin tube of magnetically soft material extending coaxially with the magnets. We have found that by proper choice of the reluctance of the field straightener material in axially extending field straighteners, it is possible also to provide the compensation at field reversals referred to above.

According to the present invention there is provided a magnet arrangement including two hollow cylindrical longitudinally magnetised permanent magnets arranged in alignment end to end with like magnet poles adjacent one another and liners of ferromagnetic material having retentivity which is small compared to the magnet material positioned coaxially within each of the two magnets on either side of the junction or gap between them, the axial reluctance of the liner material being such that the reversal of magnetic field of the said junction or gap is made more abrupt.

In preferred embodiments of the invention the liners are helices of ferromagnetic who. i

The invention will further be described with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic sketch showing a longitudinal section through a magnet arrangement for a travelling wave tube in accordance with the invention;

'FIG. 2 is an enlarged view of part of the liner of FIG. 1;

FIG. 3 shows graphs of field strength within a cylindrical permanent magnet with and without a liner according to the invention; and

FIG. 4 shows graphs of axial magnetic field plotted against distance in the region of a reversal of direction of the magnetic field for various degrees of compensation; and

FIGS. 5 and 6 show plots of the measured field strength obtained in a practical magnet arrangement such as shown in FIG. 1.

In the travelling wave tube arrangement of FIG. 1 a pair of hollow cylindrical magnets 1 and 2 (each, though not so shown, being built up of a set of annular magnets) are shown mounted in coaxial alignment on a brass tube 3 within which is shown for purposes of illustration, a travelling wave tube 4, although the travelling wave tube as such forms no part of the present invention. The magnets 1 and 2 are longitudinally magnetised and are positioned with like magnetic poles adjacent one another by an annular pole piece 5 outside the tube 3 in the same plane as an internal annular pole piece member 6 which is sandwiched between a pair of non-magnetic support members "I, made of brass for example, and having axially extending flanges to serve as seatings for respective liners 8 whose construction will be described below. To wards their distant ends the magnets 1 and 2 and the brass tube 3 are slotted to receive respective input and output waveguides 9 an'd 10 which are transversely apertured for passage of the travelling wave tube and are aligned with the brass tube 3 with the aid of brass members 11, 12 and 13 which also serve as end supports for the liners 8. Members 12 and 13 also serve as supports for liners 18 and '19 respectively, at the ends extending beyond waveguides 9 and '10. Respective pole pieces 20 and '21 terminate the electron gun and the electron collector ends of the magnet assembly, the pole piece 20 having an axial projection approaching the cathode region of the electron gun, this projection causing a peak in magnetic field at the cathode advantageous for instance, for low noise electron guns. Such a cathode field shape forms no part of the present invention but illustrates one application in a complete focusing system.

The constructions of the liners 8 is shown in the enlarged sketch of FIG, 2. A brass tube 22 seated on the liner supports carries two helices 23 of iron alloy wire, one wound on top of the other, soldered to it. The dimensions of these helices and the properties of the wire are discussed below.

The variation of magnetic field strength H along the axis of a hollow cylindrical magnet extending from points A to D is shown in the full line curve of FIG. 3. By the use of a liner of correctly chosen reluctance, the rounding-off of the curve of field strength against distance near the ends of the magnet can be avoided and, instead peaks are provided to produce a variation of magnetic field along the axis as shown in the broken curve. It will be noted that the peaks at the ends of the curve are accompanied by a reduction of field strength along the mid-portion. By correct design of the liner, curves intermediate those shown can be obtained.

In the case of the junction between two cylindrical magnets with opposite poles adjacent one another the plot of field strength obtained without any liner is shown by the full line curve a of FIG. 4. The steepness of the transition from positive to negative field direction may be adjusted by appropriate choice of the size of the pole piece aperture betweeni.e., the size of the central aperture of pole piece member 6 in FIG. 1. By the use of liners for both the magnets, in accordance with the invention, the transition can be made more abrupt, without peaks as shown in b, or, in the extreme case, with peaks as indicated by the curve 0. Whether a profile more like b or c is used in an actual device depends upon what compensation is needed to give satisfactory focusing.

The main parameters of the liner which effect the field profile are the permeability of the wire of the liner helix or helixes, the number of layers, the spacing between turns and the mean diameter of the liner. As a suitable low transverse. reluctance for field straightener use is easily obtained through the circumferential paths around the liner, only the variation of axial reluctance need be considered. An increase in the permeability of the wire, an

increase in the number of layers or a decrease in the spacing between turns, all decrease the reluctance of the axial path and thus give greater reduction in the main fiat magnetic field and a greater increase in field at the ends, i.e the curve rather than the curve b in FIG, 4. The diameter of the liner affects the abruptness on the axis of the compensation at the end of the liner, a smaller diameter causing the effect to appear on the axis in a shorter distance from the end of the liner.

In a practical embodiment of the invention of a travelling wave tube focusing magnet arrangement similar to that shown in FIG. 1, the material dimensions and magnetic properties of the magnets 1 and 2 were as follows.

The poles piece member 6 was 0.088 inch thick and has a central aperture of a Mt inch diameter. The focusing arrangement was designed for use with a low noise travelling wave tube in which the cathode is situated at a peak Length magnet 2 v of magnetic field strength. The measured variation of magnetic field along the axis of the travelling wave tube is shown in FIG. 5. The cathode position is adjacent the initial peak at A and the peak of the magnetic field in this region is produced by the re-entrant pole piece 20. Reversal of the magnetic field occurs at the point B and a relatively small peak is provided with the aid of the magnet liners adjacent the electron collector electrode position at the other end of the magnet system. The effect of the use of liners according to the invention in the neighborhood of the point B is shown on a larger scale in FIG.

' 6, where curve 0 shows the variation in field strength without liners and curve d that with the liners in position. In this example the liners 8 of FIG, 1 were provided by two concentric helices of 0.014 inch diameter alloy wire known in the trade as Cunife wound about the /2 inch diameter brass tube 22 at 71 turns per inch. The Cunife wire consists of a ferromagnetic core clad with copper to a thickness of approximately 0.001 in. to 0.002 in. The wire is close wound and suitable axial spacing between the magnetic cores is provided by the copper cladding. The gap between the adjacent liners was adjusted to provide the desired profile by choice of the thickness of the side walls of the liner supports 7. The half-width of the separation, for the magnet system under discussion was 0.140 inch. At the ends of the assembly the liners were carried the full length from the waveguide wall to the respective pole piece.

It is to be understood that the foregoing description of specific example of this invention is not to be considered as a limitation on its scope.

What is claimed is:

1. A magnetic focusing arrangement for an electron beam tube including two hollow cylindrical longitudinally magnetised permanent magnets arranged around said tube in alignment end to end and having a gap therebetween with like magnet poles adjacent one another and field straighteners of ferromagnetic material positioned coaxially around the envelope of said tube within each of the two magnets on either side of said gap, and non-magnetic metallic supports spacing said field straighteners from said envelope, the field straighteners providing low reluctance for magnetic fields transverse the axis of the magnet and relatively high axial reluctance wherein the reversal of magnetic field at said gap is selectively made more abrupt in accordance with the axial reluctance of the field straighteners.

2. A magnet arrangement as claimed in claim 1 wherein each field straightener is provided by a non-magnetic metallic cylinder and at least one wire helix of small diameter wire closely wound around said cylinder.

3. The device of claim 1, wherein said tube is a traveling wave tube and said magnets provide a magnetic field along the axis thereof, each of said magnets being magnetised from end to end in opposite directions and having an apertured pole piece of soft magnetic material in said gap and wherein said field straighteners include respective wire helices of small diameter closely Wound wire of ferromagnetic material.

4. The device of claim 2 wherein the wire comprises a ferromagnetic core having non-magnetic metallic cladding.

5. The device of claim 3 wherein said non-magnetic supports are positioned at the ends of said field straighteners and at said gap.

6. The device of claim 4 wherein the permeability of said Wire, the spacing between turns and the mean diam eter of said field straighteners are selected to provide a predetermined curve of magnetic field strength.

7. The device of claim 4 wherein said wire helix includes two concentric helices and said non-magnetic cladding provides axial spacing between adjacent cores.

8. The device of claim 5 wherein the aperture of said pole piece and the width of said nonmagnetic supports at said gap are selected to provide a predetermined curve of magnetic field strength at said gap.

References Cited UNITED STATES PATENTS 2,863,086 12/1958 Cook 3153.5 2,867,745 1/1959 Pierce 3153.5 2,911,554 11/1959 Kompfner 3l5-.-3.5

BERNARD A. GILHEANY, Primary Examiner, GEORGE HARRIS, Assistant Egraminer, 

